Sodium fluoride, NaF

The fluoride is precipitated on neutralizing hydrofluoric acid with the hydroxide or carbonate. It can also be prepared from cryolite, 3NaF,AlF3, by removing the aluminium by treatment with excess of sodium hydroxide, and extracting the undissolved sodium fluoride with boiling water.

Sodium fluoride forms colourless cubes, melting at 980°, 986° or 988° C., and boiling at 1695° C. The vapour-pressure in atmospheres corresponds with the expression

log p = -56600/4.57T + 6.299.

The density is 2.766. Sodium fluoride is the least soluble of the sodium halides, 4 parts dissolving in 100 parts of water at 15° C., and the solubility does not increase much with rise of temperature.

For the heat of neutralization of sodium hydroxide by dilute hydrofluoric acid Thomsen found 16.27 Cal. Guntz determined the heat of solution to be -0.6 Cal. at 12° C. For the heat of formation from the elements Thomsen gives 102.6 Cal.; Guntz gives 109.3 Cal. Between 15° C. and 53° C. the specific heat is 0.2675. References to investigations of the properties of aqueous solutions are appended.